Thermionic tube



June 27, 1961 R. s. SPENCER 2,990,495

THERMIONIC TUBE Filed Sept. 14, 1953 2 Sheets-Sheet 1 II: IE l IN V EN TOR ROBERT SPENCER TT'ORNEV June 27, 1961 R. s. SPENCER 2,990,495

THERMIONIC TUBE Filed Sept. 14, 1953 2 Sheets-Sheet 2 INVEN TOR ROBERT SPENCER ATTOEA/EV United States Patent 2,990,495 THERMIONIC TUBE.

Robert S. Spencer, Lbs Altos, Califi, assignor to Varian Associates, San Carlos, Calif, a corporation of California Filed Sept. 14, 1953, Ser. No. 380,068 3 Claims. (Cl. 313-618) The present invention relates to thermionic tubes and more particularly to an improved thermionic tube construction and the method of fabricating the same.

It is an object of the present invention to provide an improved tube construction such that certain operational deficiencies resulting from the intrinsic nature of such tubes and their operating environment are substantially eliminated.

More particularly, it is an object of the present invention to provide a thermionic tube construction wherein the deleterious effects of secondary electron emission are substantially eliminated.

Another object of the invention is to provide a thermionic tube construction incorporating insulation, which not only precludes arcing or other electrical breakdown throughout a Wide range of operating conditions, such as variations in pressure, temperature, humidity, and the like, but also adds to the structural ruggedness of the tube by greatly reducing the adverse effects of jolts and jars.

Additionally, it is an object to provide improvements in the method or technique for the fabrication of a thermionic tube construction so as to optimize the operational characteristics thereof.

These and other objects and the advantages of the invention will become apparent from the following description of the accompanying drawings wherein,

FIG. 1 is an elevational view of a thermionic tube embodying the present invention, a major portion of the tube being broken away to illustrate interior construction thereof,

FIG. 2 is an enlarged section taken along line 22 of FIG. 1,

FIG. 3 is an enlarged detail view of a portion of the tube shown in -FIG. 1, and

FIG. 4 is an elevation, partly in section, of another thermionic tube disposed within a cavity mold employed in its fabrication.

The thermionic tubes shown in the drawings are shown as of the klystron type and are designed to operate as oscillators. It will be appreciated from what follows that While such tubes realize particularly the advantages accruing from the present invention, other tubes, as well, are equally benefitted.

In many structural aspects, the tube shown in FIG. 1 is of conventional klystron construction and generally comprises a radio-frequency generation or resonator section disposed between an electron beam-producing section 11 and an electron collector section 12.

The electron beam-producing section 11 comprises a generally cylindrical conducting body 13 which is itself formed in sections to facilitate assembly of the tube. A ceramic stem 14 secured centrally and axially within the body 13 mounts at its upper end an electron gun 15 of conventional construction. Briefly, the gun 15 includes a cathode button 16 of dished configuration which has an electron-emissive coating applied to its upper concave surface and is secured by brazing on its under side to the rim of a cup 17 composed of a metal which is a poor conductor of heat. The cup 17 is secured to a number of tabs 18 which are formed integrally with and extend inwardly from a stepped, cylindrical sleeve 19 that is carried upon the upper end of the ceramic mounting stem 14, previously mentioned. A focusing ring 20 isbrazed interiorly of this sleeve 19 at its upper end so as to lie adjacent to the cathode 16 in axial alignment therewith. The focusing ring 20 and cathode 16 are adapted to operate at the same voltage or potential, such potential being supplied thereto by a lead 21 which is spotwelded to the sleeve 19 and extends downwardly through a central bore in the ceramic mounting stem 14 and thence through a hermetic seal 22 composed of glass which closes the lower end of the cylindrical body 13 of the electron beam-producing section 11 of the tube. This lead 21 also forms one terminal for a getter 23 disposed immediately above the glass seal 22 and for one end of a heater wire 24 which is connected to the interior of the described cathode-supporting cup 17. The heater wire 24 winds back and forth within the cup 17 so as to enable sutlicient heating of the cathode 16 and thence passes downwardly through an insulated opening 25 in the bottom of the cup for connection to a second lead 26. This plus a third lead 27 constituting the other terminal for the getter 23 both pass through the described glass seal 22 for connection to suitable potential sources (not shown). The leads 21 and 26 also pass completely through a cap 28 of insulating material, to be described in detail hereinafter.

An annular cover 29 is secured at its periphery to the upper end of the cylindrical body 13 by brazing, and to a hub portion 29a on the cover is brazed an annular disc or header 30 which forms an end wall of a chamber Within a metallic block 31 constituting the body of the resonator section 10 of the tube. The other end wall of the chamber is formed by a similar annular disc or header 32 A tubular member 33 is mounted within the chamber in axial alignment with the electron gun 15 through the medium of an integral radially-extending flange 33a which is secured by brazing at its periphery to the wall of the chamber. Thus, the chamber within the body 31 is divided into two sections which encompass opposite ends of the tubular member 33 so as to form cavity resonators 34 and 35. Resonator gaps 36 and 37 are formed by means of honeycomb grids 38 which are secured at each end of the tubular member 33 and within the central open portion of each of the described headers 30, 32.

As is now Well understood, a beam of electrons emitted from the cathode 16 is channeled by means of the focusing ring 20 so as topass through the bore in the cover 29 of the electron beam-producing section 11 and thence through each of the resonator gaps 36, 37 so that energy is transferred between the beam and the radio frequency fields in the cavity resonators 34, 35. At the first gap 36, the beam is velocity-modulated so that when the electrons reach the second gap 37 they are disposed in bunched relation whereby energy is transferred to the radio frequency field in the second cavity resonator 35. A portion of this energy is fed back to the first cavity resonator 34 by means of a small aperture 39* in the cavity-separating flange 3301 so that the present thermionic tube operates as an oscillator, as previously mentioned. The remainder of the radio frequency energy passes through a lateral opening or iris 40 in the wall of the resonator body 31 and thence to a conventional waveguide output circuit (not show) through a suitable coupling arrangement. The coupling arrangement, as shown, includes a stub section 41 of Waveguide brazed to the side of the resonator body 31 and mounting at its outer end a pair of Waveguide adapter flanges 42, 43 between which is hermetically sealed a conventional radio frequency window 44 constructed of mica.

The electron beam, after transferring its energy to the radio frequency field, passes to the collector section 12 which includes a generally cylindrical metallic block 45 having a number of radial fins 46 thereon for the dissipation of heat, the heat being produced when the 3.. electrons impinge upon the surface 47 at the base of a hollow recess formed at the lower end of the collector section 12 in direct alignment with the beam.

To .mount the collector section 12.0n. the resonator. section of the thermionic .tube, a collar 48 is brazed to the upper header 3-2r of such section and is adapted for overlapping relation with a depending cylindnicalstub 49' of the collector section 12. As shown best in FIG. '3, the collar 48- and the flange 49 are secured in structurallyrigid but electrically-isolated relation by means of solder glass (eg. #7570, Coming Glass Works) which is a glass having a relatively high lead content, a moderate degree of resiliency, great strength and excellent adherence to metallic surfaces. The solder glass is applied to the flange 49 and to the collar 48 with a procedure much like conventional soldering. The metallic elements must be indirectly heated to avoid reduction of the lead existing as an oxide within the glass. For the same reason it should be noted that the soldering connection should not be made in a hydrogen or other reducing atmosphere.

It has been found that a relatively thin layer of the solder glass, as indicated at 50 in FIG. 3, applied in the described manner betweeen the collar 48. and the flange 49, enables the operation of the resonator section 10 and. the collector section 12 at moderately differing potentials without arcing, excessive leakage, or other electrical breakown. Furthermore, as a result of the great strength and the resiliency of the solder glass, the mechanical connection between the two sections is maintained regardless of Widely varying temperature conditions and rough handling of the tube.

During tube operation, secondary electrons are emitted from the surface 47 of the collector when the electron beam impinges thereon, and many of these pass into the resonator section 10 of the tube so as to ultimately reduce its power output by the mechanism of electron loading. To diminish the number of these electrons entering the resonator section, a grid 51 of relatively great depth is secured within the collector recess in spaced relation to the electron-collecting surface 47. However, the grid 51 is itself a source of secondary electrons, though to a lesser extent. Additionally, secondary emission from the gap-forming grids 38 and other elements subject to electron bombardment has proved harmful.

Consequently, in accordance with the present invention, each of the grids 38 as well as the grid 51 and the surface 47 of the collector are provided with a coating which decidedly reduces the secondary emission ratio,

which is the ratio of the number of secondary electrons emitted from a surface to those bombardingor impinging upon that surface. The grids 38 and the. grid 51 in the present tube are formed of copper which can have a secondary emission ratio as high as 1.3 depending on the voltage of the impinging electrons, and it will be obvious that a reduction of this ratio to .a value of, say, 0.7 or the like will not only reduce the electron loading effects re sulting from secondary emission but will also preclude the possibility of the deleterious phenomenon known 'as multipactor, which occurs between two surfaces in opposed relation, one or both of which has a secondaryemission ratio greater than one, thus enabling a buildup of the number of electrons emitted from the surfaces as they pass to and fro therebetween.

The coating constitutes a thin layer of nickel carbide,

While the nickel carbide coating canbe applied to the-copper tube elements in various ways, the following method of application is considered preferable in that it enables accurate yet simple control of the thickness or depth of the carbide layer, and has produced a very adherent coating. The surface, having been cleaned, is momentarily exposed to a nickel electro-plating bath, the technique being commonly referred to as flash plating, which results in the even application to the surface ofa nickel layer of approximately 0.0005" thickness. The outer portion of the nickel layer is then converted to nickel carbide by heating the surface to 500 C. or higher 'in an atmosphere of hydrogen which has been bubbled through a hydrocarbon, preferably of high carbon-content such as benzene, gasoline, or kerosene. The element is then placed in a beaker of distilled water and violently agitated by means of a rapid influx of oxygen or other gas so that the excess carbon particles are removed from the surface leaving, as shown in FIG. 2, a thin homogeneous layer 47b of nickel carbide in tightly-adherent superposition on the nickel layer 47a which binds the nickel carbide to the surface 47 of the collector. While the precise thickness of the layers 47a, 47b is not exceptionally critical, it is preferable that they be maintained as thin as convenient so that the loss of radio frequency energy in the nickel carbide and nickel materials will be minimized. The actual loss of power resulting from the described coating has been found to be negligible. As a consequence of the tight adherence of the layers 47a, 47b, no particles are dislodged during operation to contaminate and reduce the efiiciency of the tube, which result regrettably has been observed with coatings employed fora similar purpose in the past.

It should be pointed out that any brazing operations subsequent to the assembly of the carbide-coated elements in the tube, should preferably be performed in a dry-hydrogen atmosphere to avoid reduction of the nickel car bide layer 47b. Additionally, during operation of a tube with a barium getter, evaporation of the barium and deposition thereof on the nickel-carbide has been observed with the result that the efiect of the nickel carbide in reducing the secondary emission ratio is somewhat nullifled. However, this barium deposit can be removed by temporarily operating the tube with an increased beam voltage which operation has no deleterious effect on the nickel carbide coating itself.

The nickel carbide coating may also advantageously be employed on the reflector element of a reflex klystron such as that shown in FIG. 4. This tube, as shown, comprises in conventional operating relationship a cathode 52, focusing ring '53, accelerator grid 54, a pair of closely-spaced grids forming a resonator gap 56 within a cavity 57, and a reflector element 58 which latter is supported on a stem 59 extending through and carried by a glass seal 60 at one end of the tube body 61. In normal operation of such tubes, the reflector element 58 is operated at a potential sufliciently negative that the electrons emitted from the cathode 52 and velocity-modulated in their first passage through the resonator gap 56 are, for the most part, repelled by the reflector element 58 so as to pass a second time through the gap in bunched relation to impart energy to the radio frequency field. However, in spite of the negative potential of the reflector 58, a few electrons have suflicient velocity to impinge thereon. Therefore, if the secondary emission ratio of the reflector 58 is greater than 1.0, the potential on the reflector has a tendency to move in a positive direction, particularly if the voltage source for the reflector is one of high impedance. As the potential of the reflector 58 moves in a positive direction, the electron-repelling force is reduced so that yet more electrons impinge thereon thereby causing further decrease in the negative potential of the reflector. The described phenomenon, commonly known as run away will quickly cause oscillation of the reflex klystron to cease. If continued so that more and more of the electrons bombard the reflector 58,- it will finally bedestroyed. The application of the nickel carbide coating to the surface of the reflector so as to reduce its emission ratio to a value less than 1.0 will elfectively prevent run away of the tube, and the described deleterious effects.

The accelerator grid 54, and the grids forming the gap 56 in the tube shown in FIG. 4 are coated as well as the reflector element 58 so as to reduce the electron loading efiects previously discussed. As a result, a power increase in tubes of this type has been observed to be as great as 20%.

As will be apparent, the nickel carbide coating will prove advantageous in other structures where secondary electron emission should be avoided; the collector of a traveling-wave and similar tubes and many elements of conventional low-frequency tubes are examples which may be recognized readily.

Both the tubes shown in FIGS. 1 and 4 are frequently subject to operating conditions which vary drastically and produce physical and electrical breakdowns. For example, either of the tubes shown can be employed in aircraft and in guided missiles. When a guided missile is launched, a tremendous amount of heat is generated and the tubes are of course subject to the harmful effects of such sudden temperature rise. Subsequently, as the missile attains a high altitude, and this is achieved in an extremely short span of time, the temperature drops to a sub-zero level, thus subjecting the tubes to further strain.-

Additionally, upon the attainment of such heights, a great decrease in pressure occurs which tends to facilitate arcing between tube elements of differing potential. Arcing and corona discharge between the leads in the cathode gun shown in FIG. 1 and between the reflector stem 59 and the body 61 of the tube of FIG. 4 has been noted under such operating conditions.

In order to overcome the difliculties, an insulating seal comprising a cap of insulating material constituting a silicone gum polymer commonly referred to as silicone rubber (e.g. Silastic, Dow Corning Co.) is applied in a particular manner to critical portions of the tubes.

As shown in FIG. 4, the cap, indicated at 62, is applied to the tube by a cavity-molding process. After cleaning the surfaces at the reflector end of the tube, a primer coating of ethyl orthosilicate solution (Dow Corning Primer N0. 796) is applied to those surfaces and the tube is then exposed to an atmosphere of hot water vapor for approximately fifteen minutes after which it is thoroughly air-dried. The lead wire to the reflector stem 59 is covered successively with two layers of silicone rubber tubing 63, 64 between which is interposed a thin layer of woven glass tubing 64a and the tube thus prepared is placed in one half 65 of a cavity mold which has been loaded with some previously plasticized silicone rubber. The other half 66 of the mold having also been loaded with silicone rubber is then placed and closed by means of a pressure plate (not shown) in abutting relation with the first half 65 of the cavity mold, and the end portion 67 of the mold is then positioned over guide pins 68 pro vided on each of the described halves 65, 66 of the cavity mold. The end portion 67 is held in position by means of a screw-clamp 69 which turns within a plate 70 removably secured to one half of the cavity mold. The assembled tube and mold are placed in an oven set at a temperature between 300 F. and 375 F., and the temperature of the mold allowed to rise to approximately 255 F. where this mold temperature is maintained 45 F. for a period of approximately ten minutes during which the silicone rubber bonds itself to the body of the tube and is vulcanized along with the silicone rubber tubing 63, 64 so as to form a hermetic seal between all contacting portions.

To insure that the silicone rubber completely fills the cavity mold during the described vulcanizing process, the separate portions of the mold are loaded with an amount of material slightly in excess of that required, and this excess portion, during the vulcanizing process, moves out of the main mold cavity through flash gutters indicated at 71. However, it has been discovered that the buildup of pressure within the mold during the vulcanizing process is sufliciently great to, in many instances, crack the glass seal 60 which serves to support the reflector stem 59. To avoid such cracking, a pressure release means has been incorporated in the cavity mold employed in the above described process. Such means preferably takes the form of a plug or piston 72 slidably disposed within registering bores in the end portion 67 of the mold and in the screw-clamp 69. A spring 73 rests between the piston 72 and the bottom 74 of the bore within the screw-clamp 69 and is of such strength that a pressure of approximately ten pounds per square inch does exist within the mold cavity. This pressure is the optimum required to insure that the silicone rubber material will bind securely with both the metal and glass portions of the tube during the vulcanizing process.

Upon completion of the vulcanizing process, the mold and tube are removed from the oven and exposed to an air current produced by small blower fans or the like until the mold temperature has been lowered to approximately F. The cavity mold is then removed from the tube; and after the cap 62 has been trimmed, the tube is placed back in the oven for curing. The heat control of the oven is set so that a controlled rise of 15 per hour is obtained from 250 F. until an oven temperature of 400 to 425 F. is attained, and this temperature is maintained for an additional period of six hours to complete the curing cycle.

When tubes of the type shown in FIG. 4 have been employed in guided missile operation, it was found, as previously mentioned, that due primarily to the reduction in pressure at high altitudes, arcing occurred between the highly negative reflector stem 59 and the body 61 of the tube which is disposed radially outward therefrom. By the application of the silicone rubber cap 62, an insulator far superior to the low pressure air is disposed between the two elements of the tube and, moreover, since the silicone rubber forms such a tight hermetic seal with the glass and metal surfaces on the tube, in effect there is no reduction of pressure in the described critical electrical breakdown area. In addition to the above advantages, the silicone rubber cap 62 has been found to substantially eliminate cracking of the glass seal 60 even under the extreme vibration and temperature variation encountered during the flight of a guided missile. In a like manner, the silicone rubber cap 28, applied to the cathode end of the tube illustrated in FIG. 1, provides therefor the advantages described with respect to the reflex klystron shown in FIG. 4.

Since, as indicated hereinbefore, the present invention may be evidenced in other and widely diflering embodiments without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In combination, an electron beam tube for operation under conditions of extremely high altitudes including a metallic body member forming a portion of the tube, said tube having at least one electrical conducting member extending from one end of the tube through an insulating vacuum seal secured between said body member and said electrical conducting member for applying, in use, desired electrical potentials to an electrode within the tube, and an insulating, flexible, shock-proofing, and moisture-sealing end cap permanently bonded onto said tube end comprising an in situ vulcanized and cured silicone gum polymer molded on said tube end over said electrical member for preventing electrical discharge leakage between said electrical conducting member and the metallic body member of the tube.

2. In combination, an electron beam tube for operation under conditions of extremely high altitudes including a metallic body member-forming. a portion of. the tube, said tube-having at least one.- electrical conductingmember extending from an end-ofrthe tube through an insulating vacuunrseal secured'between' thebody member and the.=-.electrica1 conducting memberr'for applying, in use, desired electrical potentials to an electrode Within the tube, and an insulating, flexible, shock-proofing, and rnoisture-sealing end cap permanently molded onto said tube end comprising an in situ vulcan-izedand cured silicone gum polymer bonded in hermetically sealed fashion on the tube end and to said electrical member and insulating vacuum seal whereby said molded cap serves to prevent corona discharge from and electrical breakdown of the electrical conducting member to said body member during operation of said electron beam tube under high humidity and high'temperature conditions.

3JIn combination, an electron beam tube for operation under conditions of extremely high altitudes including a metallic body member forming a portion of the tube, said tube having at least one electrical conducting member extending from an end thereof through an insulating vacuum seal secured between the body member and the electrical conducting member for applying, in use, desired electrical potentials to an electrode within the tube, a flexible electrical conductor fixedly secured to said electrical member and covered by an insulation tubing of silicone rubber, and an insulating, flexible, shock-proofing, and moisture-sealing end cap permanently molded onto said tube and over the secured end of said silicone rubber covered conductor comprisingan in situ vulcanized and cured silicone gum polymer bonded in hermetically sealed fashion on the tube and to the vacuum seal and the electrical'member and the silicone covered connector whereby said molded cap serves to prevent corona discharge from and electrical break-down of the electrical member to the body member during operation, of? said electron b'eamptubeunder high humidity and .high'temperature conditions while allowing flexibility o-f'said'silicone cov-' ered connector coupled to said electrical member and extending fromsaid end cap.

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